International Journal of Innovative Research and Knowledge. INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH AND KNOWLEDGE ISSN

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1 INTERNATIONAL JOURNAL OF INNOVATIVE RESEARCH AND KNOWLEDGE The study of the deposition, composition and optical properties of Cu 2 O thin films at 100 o C of NaOH solution and annealed at 250 o C for 1hour prepared by Solution Growth Technique Onwuemeka, J.I. & Nwulu, N.C. Corresponding Authur: Onwuemeka, J.I. Department of Physics, Imo State University, Owerri, Imo State Nigeria. Abstract: A research on the deposition and characterization of Copper(I) oxide thin films have been prepared by Solution Growth Technique using ammonia as complexing agent. The thin films of Cu 2 O are deposited on glass substrate at 100 o C of NaOH solution for 4hours. The deposited sample was annealed at 250 o C using Master Chef annealing machine for 1hour. The optical properties were measured using UV-1800 Series Double Beam Spectrophotometer. The composition and thickness measurements of the films were determined using Rutherford Backscattering Spectroscopy (RBS), X-ray Fluorescence (XRF) and Quantitative Analysis. The optical band gap of the deposited films of Cu 2 O is 2.61±0.05eV as annealed for 1hour at 250 o C. Other properties calculated from transmittance, using appropriate equation are absorbance, reflectance, absorption coefficient, refractive index, extinction coefficient, dielectric constant and optical conductivity. Page 59

2 I Introduction Cu 2 O thin films are good materials used as an active layer in various types of solar cells and a passive layer in solar-selective surfaces. Cu 2 O thin films are semiconductor materials with energy band gap of 2.2eV. Copper(I) oxide films have been prepared by various methods such as spraying, chemical conversion(mcdonald and Curtis,1976), chemical brightening, etching (Driver et al, 1978), an electro-deposition, electron- beam evaporation, reactive DC sputtering and chemical vapor deposition. The films in this work, are grown by solution growth technique (SGT), which creates a thin film on a solid substrate via a reaction in a liquid solution. This method is inexpensive, easy to prepare and its desired equipment are found in simple Chemistry laboratory (Tunaseviski, 2003). In this work, we report the study of the deposition, composition and optical properties of Cu 2 O thin films at 100 o C of NaOH and annealed at 250 o C for 1hour. II Experiment 20ml of 0.5M solution of copper (II) tetraoxosulphate (VI) pentahydrate (CuSO 4. 5H 2 O) reacted with 5ml of 1M solution of NH 3(aq) was added gently and a blue gelatinous precipitate was observed. It dissolved in excess aqueous ammonia solution and a deep blue solution was observed. Then 15ml of 1M sodium hydroxide (NaOH) solution maintained at the temperature of C was added and stirred until a light blue gelatinous precipitate was observed from the deep blue solution of NH 3(aq) and CuSO 4.5H 2 O. The volume ratio of the mixture CuSO 4. 5H 2 O: NH 3 : NaOH was recorded as 20:5:15. A glass substrate was inserted and supported vertically in a beaker containing the three compounds in solution and was allowed to stay without disturbance. The substrates were removed from the solution one at a time after 4.0 hours and were rinsed in distilled water and were allowed to dry in air. A blue, green and yellowish deposits were found impinged on the substrates which were suspected to be metallic oxides of copper in solution. Reaction Mechanism The reaction is mainly between copper (II) tetraoxosulphate (VI) pentahydrate (CuSO 4. 5H 2 O) in aqueous solution and the solution of sodium hydroxide (NaOH). The cation (Cu 2+ ) is supplied by Cu(II) salt and O 2- the anion is supplied by NaOH while ammonia (NH 3 ) solution is the complexing agent. The NH 3 solution is the complexing agent which controls the rate of ion-by-ion interaction and hence forming deep-blue solution of complex copper (II) ions [Cu(NH 3 ) 4 ] 2+ (tetra-ammine copper(ii) complex ion) which after a short time changed to greenish-yellow solution suspected to be copper (I) complex ions 2[Cu(NH 2 ) 2 ] +. In aqueous solution, the relative stability of Cu(I) and Cu(II) can be shifted in either direction of disproportion equilibrium; 2Cu(I) Cu(II) +Cu depending on the ligand used. Page 60

3 The reaction is shown in equation (1.1) and (1.2) below CuSO 4. 5H 2 O + 4NH 3 [Cu(NH 3 ) 4 ] 2+ + SO H 2 O deep- blue solution 2[Cu(NH 3 ) 2 ] + [Cu(NH 3 ) 4 ] 2+ +Cu(s) greenish-yellow In the solution, Cu(I) ions formed by the dissociation equilibrium, react with the OH - from the NaOH solution at C thereby forming Cu 2 O thin films with H 2 O as shown in equations (1.3) and (1.4) below. 2[Cu(NH 3 ) 2 ] + 2Cu + +4NH 3 + OH Cu + + 2OH - Cu 2 O + H 2 O..1.4 The as-deposited Cu 2 O thin films were subjected to annealing at C for one hour, a smooth yellowish-orange Cu 2 O is formed on the substrate, this is shown in equation (1.5) below. Cu 2 O +H 2 O Cu 2 O yellowish-orange Glass slide Synthetic foam Beaker Reactants FIG. 1.1: Experimental Set Page 61

4 III Results and Discussion Composition Characterization It is often necessary to determine the elements that make up the thin film samples. In this work, elemental compositions were determined by energy dispersive X-ray fluorescence (EDXRF) analysis and Rutherford backscattering spectroscopy (RBS) analysis. Rutherford backscattering spectroscopy The Rutherford backscattering spectroscopy analysis shows that the thickness sub-layers composition of Cu 2 O sample annealed at 250ºC are 96 of oxygen and 4 of copper. This shows oxygen-rich films, as a result of exposure to air and surface hydrolysis after annealing in a vacuum (Onwuemeka et al., 2014). X-ray fluorescence analysis (XRF) Then the energy dispersive X-ray fluorescence (EDXRF) analysis shows the various elements that make up the plane glass slide with different peaks. It also revealed the presence of Cu + as the main constituents of the deposited samples. Thickness measurement The thicknesses of the films in this work were measured by optical means (Fogiel, 1981). The thickness of Cu 2 O sample annealed at 250ºC is m. QUANTITATIVE ANALYSIS REPORT Tube Excitation: AG ANODE Operating at 25.0 KV Measurement date: Live time: 1000 Sec Tube Current: 0.50 Ma Method is Direct Comparison of Count rate Table 1.1: XRF Data indicating the presence of Cu + in the deposited sample. Analyzed Elements E1 Counts Compound Conc C1 Ka 1237 ± 44 Cl ± ppm K Ka 3387 ± 66 K ± ppm Ca Ka ± 142 Ca ± %w Ti Ka 155 ± 32 Ti ± ppm Mn Ka 261 ± 45 Mn ± ppm Fe Ka 3791 ± 80 Fe ± ppm Ni Ka 318 ± 56 Ni ± ppm Cu Ka 615 ± 60 Cu ± ppm Page 62

5 Figure 1.2 : X-ray fluorescence analysis of Cu 2 O thin films, indicating the presence of Cu + as well as other elements on the glass slide Observations It was observed that when 1M solution of NH 3 was added to a beaker containing copper sulphate, a light blue gelatinous precipitate was formed which was dissolved in excess NH 3(aq) solution forming a deep blue solution. When 1M solution of NaOH maintained at C was added to the solution of NH 3 and 0.5M solution of copper sulphate, the deep blue solution turned to light blue precipitate was formed. It must be stated clearly here that the chosen parameters were found largely by trial and error and we have no means of guaranteeing that they lead to the best possible optical and electrical properties (Eze,1998). From literature, copper salts, has the capability of forming complex solutions of copper (II) tetraammine complex ion and [Cu(NH 3 ) 4 ] 2+. In aqueous solution, the relative stability of Cu(I) and Cu(II) can be shifted in either direction of disproportion equilibrium; 2Cu(I) Cu(II) +Cu depending on the ligand used (Nair et al, 1988). This makes ammonia solution a suitable complexing agent for the deposition of copper(i) oxide. The concentration of Cu 2+ ion decreases with increasing concentration of complexing ions. Page 63

6 Thus the rate of reaction and the formation of precipitates are reduced, leading to a larger terminal thickness of the films (Cu 2 O). In this work, the deposition of Cu 2 O is ph-dependent. OH - ions from NaOH, did not take part in the complex formation therefore, the addition of OH - precipitated the corresponding hydrous oxide of Cu(I) which was deposited on the substrates. In the case of OH - ions taking part in the complex formation, the addition of OH - ions increase the ph value, making the complex more stable, thereby reducing the concentration of free cations. The suitable ph value in this work is between 9-11 as detected by the piston ph meter. By raising the temperature of NaOH solution at C, there occurs the dissociation of the ions of NaOH to Na + and OH -. These ions are unstable and are ready to accept opposite ions. When added to the copper complex ion, the complex acquired thermal energy and further dissociates to Cu 2+ precipitating their hydrous instead of their hydroxides which were deposited on the substrates. The reaction started almost immediately and the terminal thickness of the asdeposited films were achieved in 4hours Cu 2 O, when the temperature of NaOH solution was maintained at C. This approach, made the deposition faster and easier. The deposited films were subjected to heat treatment at 250ºC for 1hour. Optical measurements The transmittance was measured directly from the UV double beam spectrophotometer from which other optical parameters are obtained. The interpretations of Cu 2 O thin films deposited under the condition of 1M solution NaOH at 100ºC and annealed at 250ºC is given below. The wavelength dependence of optical transmittance, absorbance and reflectance in the wavelength range of 444nm to 994nm are shown in Fig.2.1, 2.2 and 2.3 respectively. The transmittance is high in the visible, UV and near-infrared regions of electromagnetic spectrum. The transmittance of the film, at 444nm wavelength increases from 22.2% as the duration of annealing temperature increases until it reaches the maximum transmittance of 78.5% at =830nm. This reveals that Cu 2 O film grown under the conditions of 100ºC of NaOH solution and the annealing temperature of 250ºC for 1hour, has transmittance, ranging from 22.2% to 78.5% (Fig.2.1), high absorbance in the UV, and low absorbance in the visible and near infrared regions of electromagnetic spectrum (Fig. 2.2). It has low reflectance from 11% to 20.3% (Fig. 2.3) in the UV, visible and near-infrared regions. The films are therefore suitable for antireflection coating applications most especially in the area of eye-glass coating, coating of windscreen of auto- vehicles, window coatings, etc. The optical band gap was evaluated using the equation of the absorption coefficient relationship for the direct transition, given by (Bube,1974), ( hv) 2 = A (hv E g ) Page 64

7 where hv is the photon energy, E g is the energy band gap, is the absorption coefficient and A is a constant that dependent on the materials. by extrapolating the linear portion of the plot 2 against photon energy (hv) at (Fig.2.10). A direct band gap of 2.61eV is obtained. This is in close agreement with the work of Ilenikhena and Okeke (Ilenikhena and Okeke, 2005) with a band gap range of 2.55eV. Due to its band gap, it is a p-type semiconductor used in electronic designs. It can be employed in the manufacture of solar cell and coating of solar panel.. The dependency of the refractive index (n= ) and the extinction coefficient (k= ) on the change in transmittance of the sample are shown in Fig.2.4 and 2.6 respectively. The sharp fall of optical conductivity o, the behaviour is caused by the dependency optical conductivity on the nature of refractive index and absorption coefficient because at the uv region, and n are greatest and diminishes as the wavelength increases. Absorption coefficient illustrated in Fig. (2.5). The absorption coefficient dropped from the peak value of 2.74x10 7 m -1 at 444nm to a minimum value of 4.59x10 6 m -1 at 994nm. The behaviour of depends on the transmittance and the thickness of the film. From Fig.2.7 the real dielectric constant decreased from 6.65 at 444nm to 3.90 at 994nm. This is as a result of long deposition time, which modifies the dielectric features of mainly dielectric thin films ( Ndukwe, 1996). Figure 2.1: Graph of transmittance against wavelength for Cu 2 O thin films. Page 65

8 Figure 2.2: Graph of absorbance against wavelength for Cu 2 O thin films. Figure 2.3: Graph of reflectance against wavelength for Cu 2 O thin films. Page 66

9 Figure 2.4: Graph of refractive index against wavelength for Cu 2 O thin films. Figure 2.5: Graph of Absorption coefficient against wavelength for Cu 2 O thin films. Page 67

10 Figure 2.6: Graph of extinction coefficient against wavelength for Cu 2 O thin films. Figure 2.7: Graph of real dielectric constant against wavelength for Cu 2 O thin films. Page 68

11 Figure 2.8: Graph of imaginary dielectric constant against wavelength for Cu 2 O thin films. Figure 2.9: Graph of optical conductivity against wavelength for Cu 2 O thin films. Page 69

12 Figure 2.10: Graph of 2 against photon energy hv for Cu 2 O thin films. IV Conclusion Copper(I) oxide films were prepared on glass substrates by solution growth technique from 1M solution of NaOH at 100ºC while copper complex was kept at room temperature. NH 3 solution was used as complexing agent. Cu 2 O thin film exhibited low absorbance and low reflectance in UV, visible and near-infrared regions of electromagnetic spectrum. Direct band gap of 2.61eV was obtained for Cu 2 O films. The other properties investigated were the optical conductivity, optical constants and absorption coefficient. The Cu 2 O film prepared under this condition is found useful as anti-dazzling coating and quality material for eye glasses, solar-thermal energy collector, selective absorbing layer, because of its high solar absorptance and low emittance (Maruyama,1998), tint films on windscreen for cars and glass windows, solar cell applications and as a p-type semiconductor material for electronic applications. As a result of its high transmittance, low absorbance and low reflectance, the Cu 2 O films can be used as anti-reflection coating on windows (heat protection) thereby cooling the interior. If this material is doped with alloyed with SnO 2 thin films,a transparent conducting electrode could be produced, which can be used in flat panel displays for electronic applications. Page 70

13 Acknowledgement We are grateful to the laboratory officials of Obafemi Awolowo University Ile-Ife in the persons of Prof. I.E. Obiajunwa of Centre for Energy Research and Development and Mr. E.A. Akinola of Central Science Laboratory for their assistance in successful realization of this work. References Bube, R.H., (1974), Electronics Property of Crystalline Solids. Academic Press, New York. Drver, P.M., McCormick, P.G., de Winter, F. and Cox, M.,(Eds),(1978) Proc. ISES, New Delhi, India, Pergamon Press, New York, p.881. Eze, F.C., (1998), Electroless Deposition of CoO Thin Films, J. Phys. D. Appl. Phys.; Vol. 31, pp 1-8. Ilenikhena, P.A. and Okeke, C.E., (2005), Comparative Studies of CoO and CuO Thin Films.Journal of the Nigeria Association of Mathematical Physics, Vol.9 pp McDonald, G,E. and Curtis, (1976) Technical Report on NASA-TM-X Murayama, T. Copper oxide Thin Films Prepared by Chemical Vapour Deposition from Copper Dipivaloylmathanate. Nair, M.T.S., Fernandez, A., Olampo, M. and Nair, P.K., (1988), Prospect of Chemically Deposited, Metal Chalcogenide Thin Films for Solar Control Applications. Solar Energy Material and Solar Cell, Vol.4, PP Ndukwe, (1996), Solution Growth, Characterization and Applications of Zinc Sulphide Thin Films; Solar Energy Materials and Solar Cells, Vol.40, pp Onwuemeka, J.I., Nwofor,O.K., Nwulu, N.C., Nwosu, I.E., Ezike, F.M., and Obzo,C.G. (2014), The Optical Study of ZnO Thin Films at Different Times of Annealing Temperatures Prepared by Chemical Bath Deposition. IOSR Journal of Applied Physics. Vol.6, pp Tunasevski,A. (2003),Semicond. Sci. Technol. 18, , Page 71